The present invention relates to an apparatus for supplying molten steel used for the continuous casting, and also to a method for continuously casting with the apparatus for supplying molten steel, which is useful to prevent an immersion nozzle clogging and to reduce slab surface defects.
As a method for continuously producing a slab, a continuous casting method is normally known, in which molten steel stored in a tundish is supplied to a top of a mold via an immersion nozzle disposed at the lower part of the tundish to form a solidified shell in the mold, and then the slab is continuously produced by withdrawing the solidified shell from a bottom of the mold.
When the molten steel, which is deoxidized with Al, is continuously cast, the Al oxide in the molten steel tends to deposit to the inner surface of the immersion nozzle, and therefore the molten steel is hindered to flow in the immersion nozzle. Therefore, when the casting is carried out using an immersion nozzle having more than one port, a symmetric flow of the molten steel in the mold cannot be obtained. In fact, when a bias flow becomes to be great, a fluid flow in the mold tends to an asymmetric flow. Accordingly, when such an asymmetric flow is generated, a mold flux on a meniscus of the molten steel in the mold is liable to be entrapped into the molten steel, and/or Al oxide or the like, deposited to the inner surface of the immersion nozzle, is peeled off and then tends to be entrapped into the molten steel.
The mold flux and Al oxide or the like entrapped in the molten steel are trapped by the solidified shell in the mold, thus slab surface defects such as powder defect and/or slag spot are liable to occur. The defects on the slab surface cause surface defects in the products, when the slab having the slab surface defects is hot-rolled.
When the amount of Al oxide deposited on the inner surface of the immersion nozzle increases, the so-called nozzle clogging takes place, so that it is difficult to continue the casting. The cleaning of the inner surface of the immersion nozzle with oxygen gas may solve the problem of nozzle clogging. Nevertheless, this deteriorates a cleanliness of the steel.
A method for purging an inert gas into molten steel passing through an immersion nozzle in order to avoid the nozzle clogging has been known (xe2x80x9cTetsu To Haganexe2x80x9d, vol. 66, S868, Iron and Steel Institute of Japan), and various methods for preventing nozzle clogging, which are applicable to the casting, have recently been proposed. For instance, a method for purging an inert gas into molten steel passing through an immersion nozzle is proposed in Japanese Patent Application Laid-open No. H4-319055, in which case, the amount (liter (Nl)/min) of the inert gas to be blown into the molten steel is adjusted in accordance with the throughput (t/min) of the molten steel passing through the immersion nozzle.
In Japanese Patent Application Laid-open No. H6-182513, moreover, a method for purging an inert gas into molten steel, wherein an AC or DC current is supplied between a porous refractory material for purging gas on the inner wall of an immersion nozzle and the molten steel passing through the immersion nozzle. In this method, the deposition of Al oxide or the like onto the inner surface of the immersion nozzle is prevented by purging the inert gas into the molten steel. At the same time, by supplying a current between the inner wall of the immersion nozzle and the molten steel, the resulting electromagnetic force applied to the molten steel promotes bubbles of the purged inert gas to remove form the refractory material for the purging gas, and thereby to reduce the size of generated gas bubbles. As a result, the size of the gas bubbles, which are trapped by the solidified shell in the mold, is reduced, thereby enabling the defects due to the gas bubbles in the slab to prevent on the surface of products, which are manufactured by hot-rolling the slab.
However, in the methods proposed in these specifications, it is found that a decrease in the amount of the purged inert gas to prevent the gas bubble trapping by the solidified shell makes it difficult to prevent the Al oxide or the like in the molten steel from depositing onto the inner surface of the immersion nozzle. On the contrary, the suppression of the deposition of the Al oxide or the like in the molten steel onto the inner surface of the immersion nozzle provides an increase in the amount of the purged gas. Thus the bubbles of the inert gas are trapped more extent by the solidified shell, thereby a greater number of the surface defects are generated in the products.
In these conventional methods, therefore, it is impossible to securely prevent the deposition of Al oxide or the like in the molten steel onto the inner surface of the immersion nozzle. Moreover, even if the deposition of Al oxide or the like in the molten steel onto the inner surface of the immersion nozzle is successfully prevented, the defects due to the gas bubbles generates on the surface of the slab, thereby resulting in the generation of the surface defects on the products. From this viewpoint, it is desirable to provide a secure and effective method for preventing the Al oxide or the like in the molten steel from being deposited on the inner surface of an immersion nozzle.
Accordingly, it is the object of the present invention to provide an apparatus for supplying molten steel, which effectively prevents Al oxide or the like in molten steel from being deposited onto the inner surface of an immersion nozzle, thereby enabling the generation of the slab surface defects due to mold flux, Al oxide or the like to be prevented, and at the same time enabling the surface defects of products produced from the slab to be effectively prevented. It is another object of the present invention to provide a method for continuously casting with the apparatus for supplying the molten steel.
In order to attain the above objects, the present inventors focused on the electrical capillarity and then developed a method for preventing the Al oxide or the like in the molten steel from being deposited on the inner surface of an immersion nozzle by utilizing the electrical capillarity. The electrical capillarity described herein implies a phenomenon in which the interfacial tension between an ion solution and an electrode immersed therein can be changed by the potential applied to the electrode. The present inventors carefully investigated the phenomenon and succeeded in finding the following features [1] to [7]:
[1] An upper nozzle, a flow control mechanism and an immersion nozzle of a continuous casting apparatus are constituted by a refractory material which exhibits either the electronic conductivity or the ion conductivity at a high temperature. As a result, the application of a potential between the molten steel and the refractory material having either the electronic conductivity and/or the ion conductivity at a high temperature during the continuous casting provides the electrical capillarity on the interfacial surface therebetween. This reduces interfacial tension, so that the depositing force of the Al oxide or the like on the surface of the refractory material is reduced, thereby making it difficult to deposit the Al oxide or the like on the surface of the refractory material.
[2] On the basis of the above presumption, an experiment was carried out wherein, employing a crucible in the laboratory use, an electrode and a refractory material rod both having a good electrical conductivity were immersed in molten steel, and a potential was applied between the refractory material rod and the electrode by supplying a current therebetween. In this experiment, it was found that the build up of the Al oxide or the like in the molten steel on the inner surface of the refractory material was reduced even in the case of a small potential, and that, irrespective of the polarity of the applied potential, an increase in the absolute value of the potential correspondingly reduced the build up of the Al oxide or the like on the surface of the refractory material.
[3] On the basis of the above experimental results, a method for preventing Al oxide or the like in the molten steel from being deposited onto the inner surface of an immersion nozzle was investigated. In order to more effectively supply a current between the refractory material having a good electrical conductivity and the molten steel passing through the immersion nozzle, the effect of the electrical insulation between paired electrodes was studied. The refractory material used for the electrical insulation normally provides a satisfactory result, if it has an electrical resistivity (specific resistance) of not less than 1xc3x97105 xcexa9xc2x7m at room temperature. However, at such a high temperature as in the molten steel, the refractory material exhibits greater ion conductivity, thereby greatly reducing the electrical resistivity and deteriorating the electrical insulation.
[4] When the electrical insulation between the paired electrodes is reduced due to the above-mentioned feature [3], no sufficient current can pass through the molten steel stream inside the immersion nozzle and thereby partial currents flow in the short circuits to materials other than the molten steel, thereby making it impossible to prevent the material such as Al oxide or the like in the molten steel from being deposited onto the inner surface of the immersion nozzle. This provides not only a waste of the supplied electric power, but also a danger of generating fine discharges due to the partial currents leaked to the exterior, as well as of both receiving an electric shock and providing the malfunction of the surrounding instruments.
[5] When a tundish is preheated or when a tundish is hot-recycled without preheating, by presetting the initial electrical resistance between paired electrodes at not less than 500xcexa9 just before the molten steel is supplied to the tundish, sufficient current can flow in the molten steel passing through the immersion nozzle during the whole casting period from the start to the end of casting, and making it possible to prevent the currents from flowing into the short circuits to the materials other than the molten steel. The above-mentioned term xe2x80x9cduring the period from the start to the end of castingxe2x80x9d is generally 60 to 500 min., dependent on the type of the continuous casting machine, the size of slab, the casting rate, the number of heats in continuous casting and so on.
[6] It is preferable that the electrical resistance in the period from the start to the end of casting, said resistance being calculated from the current and voltage between the paired electrodes, is less than {fraction (1/10)} of the initial electrical resistance between one electrode and the other electrode, which is the value of the resistance either at the end of preheating before the molten steel is supplied to the tundish or before the molten steel is supplied to the tundish if the tundish which is once used for casting is recycled without preheating.
[7] In other words, the feature [6] implies that the electrical resistance calculated by the current and voltage between the paired electrodes in the electric circuit constituted by the molten steel stream inside the immersion nozzle gradually increases in the course of the casting. If the electrical resistance during the casting further increases after the end of the gradual increase, no sufficient current can pass through the molten steel stream inside the immersion nozzle, and therefore the partial currents begin to flow to the short circuits constituted by the materials other than the molten steel. By controlling the electrical resistance in the course of casting to the end of casting in such a way that it can be set to be less than {fraction (1/10)} of the initial electrical resistance between one electrode and the other electrode just before the molten steel is supplied to the tundish, the electrical current can be sufficiently passed through the molten steel stream inside the immersion nozzle, thereby making it possible to suppress the partial currents to short circuits constituted by the materials other than the molten steel.
Accordingly, the present invention is completed on the basis of the above-mentioned features and it is characterized by an apparatus for supplying molten steel defined by the following structural arrangement (1) or (2) as well as by a continuous casting method defined by the following structural arrangements (3) to (7):
(1) An apparatus for supplying molten steel used for the continuous casting, characterized in that said apparatus comprising a tundish for storing the molten steel, an upper nozzle disposed in the bottom of the tundish, a flow control mechanism for controlling the flow rate of the molten steel from the tundish into a mold and an immersion nozzle for supplying the molten steel into the mold, wherein providing a pair of electrodes and a power supply connected thereto, and forming the inner surface, being in contact with the molten steel, of one of the upper nozzle, the flow control mechanism and the immersion nozzle, by a refractory material having a good electrical conductivity at a temperature not less than the melting point of steel, wherein the one electrode of the paired electrodes is disposed in one of the tundish, the upper nozzle, the flow control mechanism and the immersion nozzle in such a way that the one electrode reaches the inner space of thereof and is in contact with the molten steel, wherein disposing the other electrode in a part formed by the refractory material having a good electrical conductivity.
(2) In the apparatus for supplying molten steel having the above-mentioned structural arrangement (1), it is preferable that the refractory material having a good electrical conductivity has a conductivity of not less than 1xc3x97103 S/m at the melting point of steel and/or comprises an alumina graphite. Moreover, in the molten steel supplying apparatus having the above structural arrangement (1), it is preferable that an insulating element is interposed between the one electrode and the other electrode and/or that a gas purging part is provided in one of the upper nozzle, the flow control mechanism and the immersion nozzle which have no electrode.
(3) A continuous casting method, characterized in that supplying a molten steel stored in a tundish into a mold using the apparatus for supplying molten steel having the above-mentioned structural arrangements (1) and (2), whereby supplying an electric current between the inner surface of the upper nozzle, the flow control mechanism and the immersion nozzle in which the other electrode of the paired electrodes is disposed and the molten steel passing through the inside thereof.
(4) A continuous casting method, characterized in that, in the case of supplying a molten steel stored in a tundish into a mold using the apparatus for supplying a molten steel, having the above-mentioned structural arrangements (1) and (2), whereby setting the electrical resistance between the one electrode and the other electrode to be not less than 500xcexa9, either at the end of preheating the tundish before the molten steel is supplied to the tundish, or before the molten steel is supplied to the tundish if the tundish which is once used for casting is recycled for casting without preheating.
(5) In the continuous casting method having the above-mentioned structural arrangement (4), it is preferable that the electrical resistance determined from the current and voltage applied between the one electrode and the other electrode during a period from the start and to the end of casting is set to be less than {fraction (1/10)} of the electrical resistance between the one electrode and the other electrode, either at the end of the preheating of the tundish before the molten steel is supplied to the tundish, or before the molten steel is supplied to the tundish if the tundish which is once used for casting is recycled for casting without preheating.
(6) In the continuous casting method having the above-mentioned structural arrangements (3) to (5), it is preferable that an electrical current is supplied at a current density of not less than 0.001 A/cm2 and less than 0.3 A/cm2 and/or that the applied voltage is not less than 0.5 V and not more than 100 V.
(7) A continuous casting method, characterized in that, in the case of supplying a molten steel stored in a tundish into a mold using the apparatus for supplying molten steel, having the above-mentioned structural arrangements (1) and (2), whereby forming at least the immersion nozzle by a refractory material having a good electrical conductivity at a temperature not less than the melting point of steel, disposing the other electrode therein, applying a negative potential is applied to the immersion nozzle and supplying a DC current between the immersion nozzle and the molten steel passing through the inside of the immersion nozzle to prevent the immersion nozzle from being stopped up.
In accordance with the present invention, the material for producing the immersion nozzle and the like is selected from refractory materials having a good electrical conductivity at a temperature not less than the melting point of steel. This is due to the necessity of flowing the electrical current between the refractory material and the molten steel. In the following description, the expression xe2x80x9ca material having a good electrical conductivity at a temperature not less than the melting point of steelxe2x80x9d will be sometimes abbreviated by an expression xe2x80x9ca material having a good electrical conductivityxe2x80x9d.
The expression xe2x80x9cat the end of the preheating of the tundish before the molten steel is supplied to the tundishxe2x80x9d, which is defined in the above structural arrangements (4) and (5) according to the present invention, means the following:
The refractory materials disposed in the tundish, as well as the refractory materials included in the upper nozzle, the gate for controlling the amount of the molten steel to be supplied into the mold, the immersion nozzle and the like are normally preheated by the combustion gas, before starting the continuous casting by supplying the molten steel into the tundish. This is due to the fact that the refractory materials may be damaged by a thermal shock in the case of pouring the molten steel into the tundish and mold, and that the initially supplied molten steel solidifies on the refractory material, and such an undesirable damage must be avoided. In this case, the surface temperature of these refractory materials at the end of preheating should be typically 800 to 1,300xc2x0 C. However, the target temperature on the surface of the refractory materials after preheating depends on the casting work conditions, such as the capacity of the tundish, the time between the start of supplying the molten steel into the tundish and the start of supplying the molten steel into the mold, and others.
The electric circuit between the paired electrodes at the end of preheating in the state of the molten steel being not yet supplied to the tundish includes the refractory materials disposed in the tundish, the refractory materials constituting the upper nozzle, the gate and the immersion nozzle, and a steel structure for supporting these refractory materials. The electrical resistance of the refractory materials and the steel structure normally decrease with the increase of the temperature.
From these facts, the expression xe2x80x9cthe electrical resistance between the one electrode and the other electrode in the end of preheatingxe2x80x9d implies an electrical resistance between the one electrode and the other electrode in an electrical circuit, which may be constituted by refractory elements in a tundish heated at a target surface temperature, refractory such as upper nozzle, a gate and an immersion nozzle, and a steel construction for supporting these refractory materials, so that it implies the electrical resistance minimized just before starting to supply the molten steel into the tundish. In the following description, this electrical resistance will be sometimes denoted by xe2x80x9can initial electrical resistancexe2x80x9d.
Similarly, the expression xe2x80x9cthe electrical resistance between the one electrode and the other electrode before supplying the molten steel into the tundish when the tundish which is once used for casting is recycled for casting without preheatingxe2x80x9d, which is defined in the above structural arrangements (4) and (5) according to the present invention, implies the following facts:
In recent years, from the viewpoint of reducing the energy cost, the so-called hot tundish recycling, in which the tundish is recycled without cooling, is employed. In this case, two methods can be applied; in the one method, the tundish is preheated, and in the other method, new molten steel is supplied into the tundish without preheating. In the case of non-preheating, the surface temperature of the refractory materials in the tundish is 1,000 to 1,400xc2x0 C. The above-mentioned electrical resistance means the electrical resistance between the one electrode and the other electrode in an electric circuit which is constituted by the above-mentioned refractory materials and the steel structure at such a high temperature, and therefore it means the electrical resistance just before the molten steel is supplied to the tundish. In other words, it means the initial electrical resistance.
The expression xe2x80x9cthe electrical resistance which is determined by the current and voltage between the one electrode and the other electrode during the time interval from the start to the end of castingxe2x80x9d defined in the above structural arrangement (5) according to the present invention means an electrical resistance between the one electrode and the other electrode in an electrical circuit of the molten steel supplied into the tundish. Such an electrical resistance in the electrical circuit of the molten steel increases with the increase of the casting time. Hereafter, this electrical resistance is denoted in some cases by xe2x80x9cthe electrical resistance during the castingxe2x80x9d.